1. Field of the Invention
The present invention relates to a method of fabricating SOI (Silicon on Insulator) wafer and, more particularly, to a method of fabricating SOI wafer whose silicon layer formed on insulator has uniform thickness and low impurity concentration.
2. Description of the Prior Art
Two techniques are known as a method of fabricating SOI wafer by a conventional art. One is SIMOX (Separated by Implanted Oxygen) technique, another is a face to face attaching technique of two silicon substrates whose oxide film is formed on the surface thereof.
Recently, silicon wafer is widely used as the material for fabricating semiconductor electronic devices. There are a lot of electronic devices such as memory devices and integrated circuit devices using the silicon single crystal wafers. These electronic devices are such that a partial electrical insulation is necessary in their fabrication process. However, the silicon doesn't have the insulating characteristics in material properties. Thus, the PN junction of silicon substrate is used for the respective partial insulation in the silicon semiconductor process for fabricating the electronic device.
Since the SOI structure has been recently expected to solve the problem of insulation, the SOI wafer has been developed as a substrate for the next generation electronic device. The SOI wafer can solve the problems such as the parasitic electrostatic capacitance and incomplete insulation between individual devices that the prior silicon substrate faces in integration of the electronic devices.
The SIMOX technique is to fabricate SOI substrate by means of generating a buried nonconducting layer of SiO2 inside of the pure silicon wafer. After the oxygen ions of high energy are implanted in the certain depth from the surface of the silicon wafer, and then the implanted oxygen atoms and the silicon atoms inside of the wafer are reacted together through the heat treatment to be transformed into SiO2.
However, there are shortcomings because a large area silicon wafer is used in the oxygen ion implantation process. It is difficult to control uniformly the thickness of the surface silicon and the generated SiO2 layer. Furthermore, the concentration of impurities such as point defects is high in the surface silicon layer because of using the oxygen ion of high energy.
The another technique for fabricating SOI substrate is to form SiO2 layer on the surface of a silicon wafer by the oxidation process, attach face to face the SiO2 layers of these two oxidized silicon wafers, and remove certain amount of the silicon in one side of wafer. However, since the silicon removal process of the large area is necessary in this fabrication process, it is difficult to uniformly control the thickness of silicon left.
That is, the thickness of silicon remained as the active layer in such SOI wafers must be very uniform and the concentration of impurities must be low. However, it is difficult to fabricate a silicon layer whose thickness is uniform and impurity concentration is low by the prior arts.